Both cell division and programmed cell death are thought to reflect competition for survival among normal cells. Dynamic equilibrium between proliferation and death may be essential to growth control, permitting both homeostasis under varying growth conditions and changes in tissue size and shape at particular developmental stages. In this view cell competition and the parameters that regulate it should be responsible for defining size and shape. Here it is proposed to identify the molecular mechanisms of cell competition in vivo by performing genetic screens for Drosophila mutations that prevent cell competition from occurring. Identification of such genes will provide a basic understanding of size and shape regulation during growth, in developmental disorders including cancer and neurodegenerative diseases, and of morphological changes during evolution.